Fine spherical particles of aluminium oxide have a variety of uses, in particular within the field of ceramic materials.
As an example fine spherical particles of aluminium oxide may be used as pigments, abrasives and polishing agents, in refractory and fire-resistant products, in ceramics, as catalyst materials, or as fillers. Depending on the purity of the aluminium oxide the particles may also be used as high-performance structural ceramics, as high-performance polishing agents (for semiconductors), as starting materials for optical and functional ceramics, bioceramics and so forth.
In the prior art aluminium oxide has been prepared by a number of methods, a couple of which will be mentioned below.
U.S. Pat. No. 4,291,011 describes a method for production of aluminum oxide by continuously reacting high purity metallic aluminium, either solid or liquid, with high purity oxygen, present in stoichiometrically excessive amount, within a vertically oriented cylindrical vessel closed at the top and open at the bottom and having cooled side walls on which there forms a layer of solid aluminum oxide. Newly formed aluminum oxide deposits as a liquid film on the inside surface of this layer and flows downwardly thereover by gravity to the open bottom of the container and then falls to a collecting vessel below, solidifying either during this fall or after impingement on a moving metallic surface. An annular array of burners, e.g. oxy-hydrogen burners, limits downward growth of the layer below the vessel, and fosters detachment of the downwardly flowing film in the form of drops to fall to the collection vessel. This process does not give spherical submicron particles of aluminium oxide.
Canadian patent number 2110961 describes a process for preparing aluminum oxide particles, an aluminum oxide powder prepared according to the process and its use. The process comprises the following steps:
introduction of an aluminum carrier, such as Al or Al2O3 into a furnace unit, heating the aluminum carrier, reduction of the aluminum carrier, unless it is introduced as metallic aluminum, into metallic aluminum and/or aluminum carbides (including aluminum oxycarbides), increasing the furnace temperature to a value at which the metallic aluminum or the aluminum carbides evaporate, subsequent oxidation of the metallic aluminum or aluminum carbides into aluminum oxide in a gas flow, and introduction of the gas flow into a filter, wherein the temperature, the atmosphere, and the hold time of the aluminum oxide particles in the gas flow are adjusted according to the desired particle size. A disadvantage of this process is the use of carbon, which may lead to formation of impurities in the form of carbide and oxycarbides.
The object of the present invention is thus to provide an alternative and improved process, avoiding the disadvantages of the former processes, for the preparation of very fine and pure aluminium oxide particles, in the submicron range (<1.0 μm). An object is also to provide a process which may be carried out in a relatively inexpensive manner.